1. Field of the Invention
This invention relates to an improved electrophotographic photoconductor.
2. Discussion of Background
Recently, organic photoconductive materials, which are relatively inexpensive and cause no enviromental pollution, have been widely used as photoconductors of electrophotographic copying machines.
As an organic electrophotographic photoconductor, there have been known photoconductive resin type photoconductors, typically a photoconductor comprising polyvinyl carbazole (PVK), charge-transport complex type photoconductors, typically a photoconductor comprising polyvinyl carbazole 2,4,7-trinitro-fluorenone (PVK-TNF), pigment-dispersed type photoconductors, typically a photoconductor in which phthalocyanine is dispersed in a binder resin, and function-separated type photoconductors such as a photoconductor consisting essentially of a charge-generating material and a charge-transporting material. Of these, the last one is currently attracting much attention.
When a high-photosensitive photoconductor of the function-separate type is applied to the Carlson process, it exhibits low chargeability and weak electric charge retention (large dark decay). Furthermore, the chargeability and electric charge retention properties are drastically deteriorated in the course of repeated and continuous use, causing uneven image density and fogging. In addition, toner particles are unfavorably deposited on the background when reverse development is performed.
In general, the chargeability of the high photo-sensitive photoconductor is reduced due to pre-exposure fatigue. Such fatigue is chiefly caused by the light absorbed by the charge-generating material contained in the photoconductor. It is therefore considered that the longer the period in which the electric charges generated by the light absorption remain in a migrational state in the photoconductor and the greater the number of the generated electric charges, the greater the reduction in the chargeability of the photoconductor. Even if the photoconductor is electrically charged while the electric charges generated by light-absorption are in a residual state, the surface potential will not be elevated until the residual electric charges are dissipated. This is because the electric charges at the surface of the photoconductor are neutralized with the residual carriers which migrate in the photoconductor. The rise of surface potential is thus delayed corresponding to the pre-exposure fatigue, so that the apparent surface potential is lowered.
In an attempt to overcome the above shortcomings in the prior art, the following protective layers have been proposed: a protective layer in which aluminous fibers are dispersed as disclosed in Japanese Laid-Open Pat. Application 55-45024; protective layers in which Al.sub.2 O.sub.3, SiO.sub.2, SiC, SiO.sub.2 Al.sub.2 O.sub.3 3 and B4C are each dispersed as disclosed in Japanese Laid-Open Pat. Applications 56-38054, 56-99347 and 57-165848; a protective layer in which SnO.sub.2 and Sb.sub.2 O.sub.3 are dispersed as disclosed in Japanese Laid-Open Pat. Application 58-121044; a protective layer in which CuI is dispersed as disclosed in Japanese Laid-Open Pat. Application 59-159; a protective layer in which TiO.sub.2 is dispersed as disclosed in Japanese Laid-Open Pat. Application 60-75842; protective layers in which indium oxide is dispersed as disclosed in Japanese Laid-Open Pat. Applications 57-30846 and 57-154250; and a protective layer coated with MgF.sub.2, SiN.sub.4, Al.sub.2 O.sub.3 or ZrO.sub.2 by means of sputtering as disclosed in Japanese Laid-Open Pat. Application 58-59459.
Furthermore, the following intermediate layers have also been proposed: an intermediate layer in which an organic titanium compound is dispersed as disclosed in Japanese Laid-Open Pat. Application 58-18637; intermediate layers in which organic metallic compounds are dispersed as disclosed in Japanese Laid-Open Pat. Applications 58-60748 and 59-46653; an intermediate layer in which zirconium alkoxide or zirconium alkoxide with a silane coupling agent is dispersed as each disclosed in Japanese Laid-Open Pat. Applications 59-223438 and 58-121043; intermediate layers in which thermoplastic polycarbonate resin, acrylate resin and methacrylate resin are each dispersed as disclosed in Japanese Laid-Open Pat. Applications 60-12552, 60-142341 and 60-117561.
The above-mentioned protective layers and intermediate layers, however, cannot sufficiently prevent the reduction in chargeability caused by the repeated and continuous use of the photoconductors, the delay in the rise of the surface potential thereof, or the considerable change in the residual electric potential.